(1) Field of the Invention
The present invention relates to the field of microcircuit fabrication. More specifically, the present invention relates to a method and apparatus for microlithographic printing.
(2) Art Background
Microlithographic Process
FIG. 1A illustrates a microlithographic printing process. A pattern transfer tool 130 is interposed between a source of radiant energy 105 and a semiconductor wafer 101. The semiconductor wafer 101 includes a film 110 disposed between a semiconductor substrate 100 and a layer of photoresist 120. Herein, film 110 is described as being a layer of polysilicon 110, but may also be a metal, an interlayer dialectric (ILD) or any other film in which features are to be patterned. The pattern transfer tool 130, which may be a mask or reticle, includes a pattern 135 of opaque regions 131 and 132 disposed thereon. Radiant energy 105 striking the pattern transfer tool 130 is blocked at opaque regions 131 and 132, but otherwise passes through the pattern transfer tool to irradiate regions in the photoresist layer 120 of the wafer 101. As FIG. 1A illustrates, the effect of exposing the wafer 101 to radiant energy 105 through the pattern transfer tool 130 is to irradiate regions of the photoresist layer 120 in a pattern defined by the opaque regions 131 and 132 on the pattern transfer tool 130. The features defined by opaque regions 131 and 132 on the pattern transfer tool 130 are said to be "printed" on the wafer surface and the process of exposing the photoresist-coated wafer 101 to radiant energy 105 is referred to herein as "printing".
The photoresist of layer 120 includes a photoactive compound which chemically reacts when exposed to radiant energy, typically actinic light. In one type of photoresist, known as negative photoresist, irradiation induces a "crosslinking" chemical reaction to render the irradiated regions in the photoresist insoluble in a solvent called "developer". After application of developer, referred to as the "development step", non-irradiated regions 121 and 122 of photoresist layer 120 are removed. Consequently, the pattern formed in the negative photoresist layer is the negative of the pattern defined by the opaque regions 131 and 132 of the pattern transfer tool 130.
FIG. 1B illustrates the state of the semiconductor wafer 101 after irradiation and development of a positive photoresist. Irradiation of positive photoresist induces a chemical reaction to render the irradiated regions of the positive photoresist soluble in developer. As FIG. 1B illustrates, after the development step, only regions 121 and 122 remain of the photoresist layer 120 depicted in FIG. 1A. Thus, the pattern formed in the positive photoresist is a positive image of the opaque regions 131 and 132 on the pattern transfer tool 130 of FIG. 1A. Due to resolution limitations inherent in presently available negative photoresist materials, positive photoresist is used almost exclusively in microlithographic printing of submicron features. For this reason, the present invention is discussed primarily in terms of microlithographic printing in positive photoresist. It will be appreciated, however, that the apparatus and method of the present invention may be used to print features in either positive or negative photoresist.
After the wafer has been developed to the state shown in FIG. 1B, an etching compound is applied to etch away the regions of polysilicon layer 110 not covered by the remaining regions of photoresist 121 and 122, thereby revealing regions of the substrate 100. The result, shown in FIG. 1C, is a reproduction of the image on the pattern transfer tool in the polysilicon layer (element 110 of FIGS. 1A and 1B). At this point, the remaining photoresist regions 121 and 122 can be stripped away leaving only features 111 and 112 in what was the polysilicon layer 110 of FIGS. 1B and 1A. When surrounded by wells of appropriately doped substrate, the polysilicon features 111 and 112 define gates of field-effect transistors (FET).
The Critical Dimension
The minimum width of an isolated line that can be printed by a microlithographic process, referred to as the "critical dimension", is one of the process's most important characteristics. The smaller the isolated linewidth, the narrower the FET gate that can be formed. Since a FET's maximum switching frequency and area-density both increase as the gate is narrowed, the ability to reduce the width of the isolated line translates directly into the ability to produce faster, more densely populated integrated circuits.
Microlithographic Printing Machines
Microlithographic printing is performed by a machine known as an "aligner" for its ability to align a wafer to a pattern transfer tool. The manner in which the wafer is aligned to the pattern transfer tool depends on whether the pattern transfer tool is a mask or a reticle. If the pattern transfer tool is a mask, the wafer is aligned to the mask once, then all of the wafer die are exposed at the same time. However, if the pattern transfer tool is a reticle, the reticle pattern is transferred to only one (or a few) wafer die per exposure. After each exposure, a stage to which the wafer is mounted is stepped to align the next die for exposure through the reticle and the print is repeated. Such aligners are referred to as "step and repeat aligners," "steppers" or, in some cases, "scanners" (a scanner is stepper in which exposure through the reticle is achieved by scanning the reticle with the radiant energy source. Herein, the term stepper is used to mean any step and repeat device, including a scanner). Since the step and repeat print process must be performed potentially as many times as there are die in the wafer, the step and repeat process is considerably more time consuming than a single-exposure mask process. However, since a much smaller area of the wafer is printed per exposure, the reticle pattern may be an enlargement of the pattern to be printed and therefore easier to fabricate. Also, stepping each die allows wafer distortion to be compensated on a die by die basis.
Modern microlithographic printers are said to be diffraction limited. That is, diffraction of light passing through clear regions within a pattern transfer tool such as a mask or reticle, and not defects in the elements of the optical path, limit the minimum isolated linewidth achievable with a given machine. Radiation spreads out from the individual slits on the pattern transfer tool to regions on the wafer that are not intended to be exposed. At some point, the slits, or clear regions, of the pattern transfer tool become so close together that their resultant images cannot be resolved on the wafer surface. The minimum distance between pattern transfer tool features has been shown to be proportional to the wavelength of light used to irradiate the wafer and inversely proportional to the numerical aperture (NA) of the objective lens used to collect light diffracted from the pattern transfer tool and focus it onto the wafer.
Reducing minimum feature size by increasing the NA of the objective lens is difficult and involves tradeoffs in other aspects of the fabrication process. The depth of focus of a lens, for example, is inversely proportional to the square of the lens NA so that increased NA results in exponentially decreased depth of focus. If the depth of focus becomes too small, variations in wafer flatness can render regions of the wafer outside the focal plane. Aside from the depth of focus problem, increased lens NA generally requires a larger diameter lens, meaning that lens NA can be increased only up to a practical limit.
Reducing minimum feature size by shifting the irradiating wavelength deeper into the UV region also presents difficulties. As the irradiating wavelength is shortened, the brightness of existing light sources is severely reduced and the optical elements in the printer absorb more of the energy passing through them. As a consequence, the total energy incident on the resist is reduced. Longer exposure times are required resulting in reduced throughput.
Since the irradiating wavelength and the lens NA are generally fixed for a given printer, the printer is subject to rapid obsolescence as feature sizes shrink. Consequently, printers incorporating the latest technology and purchased at enormous cost may become unable to support fabrication of flagship products within just a few years.
Systematic Difference in Critical Dimension
It is common for features printed using a particular microlithographic printing process to exhibit systematic differences in critical dimension. The systematic critical dimension difference is usually most noticeable between horizontally and vertically aligned features and typically ranges from 10 to 30 nm. For features having critical dimensions greater than roughly one micron, a 10-30 nm critical dimension difference can be ignored. As feature critical dimensions drop below one micron, however, a 10-30 nm vertical/horizontal critical dimension difference begins to be substantial. Since, as stated above, device characteristics are fundamentally affected by the critical dimension of polysilicon gates, a substantial difference in critical dimension results in a noticeable performance inequality between vertically and horizontally aligned features.
It would be desirable, therefore, to provide a method and apparatus for printing horizontally and vertically aligned features on a semiconductor wafer having reduced, substantially equal, critical dimensions.